An environmental group that analyzed the drinking water in 35 cities across the United States, including Bethesda and Washington, found that most contained hexavalent chromium, a probable carcinogen that was made famous by the film "Erin Brockovich."

The study, which will be released Monday by the Environmental Working Group, is the first nationwide analysis of hexavalent chromium in drinking water to be made public.

It comes as the Environmental Protection Agency is considering whether to set a limit for hexavalent chromium in tap water. The agency is reviewing the chemical after the National Toxicology Program, part of the National Institutes of Health, deemed it a "probable carcinogen" in 2008.

The federal government restricts the amount of "total chromium" in drinking water and requires water utilities to test for it, but that includes both trivalent chromium, a mineral that humans need to metabolize glucose, and hexavalent chromium, the metal that has caused cancer in laboratory animals.

Last year, California took the first step in limiting the amount of hexavalent chromium in drinking water by proposing a "public health goal" for safe levels of 0.06 parts per billion. If California does set a limit, it would be the first in the nation.

Hexavalent chromium was a commonly used industrial chemical until the early 1990s. It is still used in some industries, such as in chrome plating and the manufacturing of plastics and dyes. The chemical can also leach into groundwater from natural ores.

The new study found hexavalent chromium in the tap water of 31 out of 35 cities sampled. Of those, 25 had levels that exceeded the goal proposed in California.

The highest levels were found in Norman, Okla., where the water contained more than 200 times the California goal. Locally, Bethesda and Washington each had levels of 0.19 parts per billion, more than three times the California goal.

The cities were selected to be a mix of big and smaller communities and included places where local water companies had already detected high levels of "total chromium."

"This chemical has been so widely used by so many industries across the U.S. that this doesn't surprise me," said Erin Brockovich, whose fight on behalf of the residents of Hinkley, Calif., against Pacific Gas & Electric became the subject of a 2000 film. In that case, PG&E was accused of leaking hexavalent chromium into the town's groundwater for more than 30 years. The company paid $333 million in damages to more than 600 townspeople and pledged to clean up the contamination.

"Our municipal water supplies are in danger all over the U.S.," Brockovich said. "This is a chemical that should be regulated."

Max Costa, who chairs the department of environmental medicine at New York University's School of Medicine and is an expert in hexavalent chromium, called the new findings "disturbing."

"At this point, we should strive to not have any hexavalent chromium in drinking water" or at least limit the amounts to the level proposed by California, Costa wrote in an e-mail.

Hexavalent chromium has long been known to cause lung cancer when inhaled, but scientists only recently found evidence that it causes cancer in laboratory animals when ingested. It has been linked in animals to liver and kidney damage as well as leukemia, stomach cancer and other cancers.

The American Chemistry Council, which represents the chemical industry, says the California goal is unrealistic because some water supplies have naturally occurring hexavalent chromium that is higher than .06 parts per billion.

In a written statement, the group's senior director, Ann Mason, said that "even the most sophisticated analytical methods used by EPA are not able to detect the extremely low levels that California wants to establish."

The group supports a "uniform, national standard for hexavalent chromium in drinking water, based on sound science," Mason wrote. "Research is underway to provide EPA with critical data that will allow for a more informed risk assessment of hexavalent chromium. This data will be complete by mid-2011. Given the potential impact on drinking water supplies, EPA should incorporate this data in its assessment."

Brendan Gilfillan, an EPA spokesman, said that the agency was aware of the new study by the Environmental Working Group and that the findings will be considered as the agency reviews total chromium in drinking water, work that is expected to be completed next year.

Ken Cook, president of the Environmental Working Group, said that water utilities across the country are resistant to the regulation.

"It's not their fault. They didn't cause the contamination. But if a limit is set, it's going to be extraordinarily expensive for them to clean this up," Cook said. "The problem in all of this is that we lose sight of the water drinkers, of the people at the end of the tap. There is tremendous push-back from polluters and from water utilities. The real focus has to be on public health."